1. Field of the Invention
This invention relates to a torque-indicating wrench, more particularly to an electrical torque-indicating wrench with a replaceable strain gage unit mounted to a wrench body for measuring a torque applied to a workpiece with a high degree of precision.
2. Description of the Related Art
U.S. Pat. Nos. 3,970,155, 4,006,629, 4,522,075, 4,669,319 and 4,976,133 disclose electrical torque-indicating wrenches that generally have strain gages attached to a lever arm proximate to a head for measuring torque. The head is suited to engage and rotate a workpiece by applying a force to the lever arm. The strain gages detect elastic strains and changes in electrical resistances, translate the changes into an electrical signal, and, in cooperation with a processing circuit which includes a Wheatstone bridge, an amplifier, a recorder, a microprocessor, an output unit, etc., determine a value of torque applied to the workpiece. Strain (ε) is related to a bending moment (M) through the relationship:   ɛ  =            M      *      y              E      *      I      
When an object with a modulus of elasticity (E) and a moment of inertia (I) relative to a neutral axis is subjected to a bending moment (M), the strain (ε) is directly proportional to a distance (y) between the measured point and the neutral axis. In the aforementioned prior art wrenches, the strain gages are attached to a lateral surface of the lever arm away from the neutral axis so as to produce a larger strain value, based upon which the processing circuit can calculate the torque precisely. However, a relatively large area of the lateral surface of the lever arm to which the strain gages are attached has to be machined with high precision so as to facilitate attachment of the strain gages, thereby resulting in higher manufacturing cost. Furthermore, conventional strain gages are not replaceable once they are broken or damaged.
Referring to FIGS. 1 and 2, a conventional electrical torque-indicating wrench is shown to include a wrench body 1 having a head 102 and a handle 101 extending from the head 102. The handle 101 and the head 102 respectively have first and second mounting slots 1011, 1021 for receiving a strain detecting unit 2 and a display unit 3. The strain detecting unit 2 has a substrate 201 with a profile the same as that of the first mounting slot 1011. The substrate 201 has a recess 2011 for receiving a strain gage 202. When a torque producing force is applied to the handle 101, stretching deformation of the strain gage 202 results in a change in electrical resistance, which is detected and translated into an electrical signal. The strain detecting unit 2 can be replaced once the strain gage 202 is broken. However, since the profile of the substrate 201 has to be same as that of the first mounting slot 1011, the strain gage 202 is placed in close proximity to the neutral axis (X), thereby reducing the distance (y). Therefore, the strain (ε) in the strain gage 202 is relatively small so that the change in electrical resistance is too small to permit precise torque measurement. Furthermore, although a signal output corresponding to the change in resistance can be amplified by amplifiers, the errors in the signal will be amplified at the same time so that a precise torque value cannot be obtained.